Exemplary embodiments of the invention generally relate to a rotary wing aircraft, and more particularly, to a higher harmonic vibration control system therefor.
The reduction of vibrations is a primary goal in rotary-wing aircraft design. Such vibrations may contribute to crew fatigue, increased maintenance, and operating costs. A major cause of such vibrations is periodic aerodynamic loads on the rotor blades.
An effective method of reducing rotor-blade induced vibrations is to control the harmonic airload at the source, i.e., on the rotor blades. For an N bladed rotor, harmonic loads at a frequency rate of (N−1) per revolution, N per revolution, and (N+1) per revolution are transmitted to the rotor hub. All three harmonic load frequencies in the rotating rotor frame of reference result in fuselage vibration in the non-rotating frame of reference at the frequency ratio of N/revolution (hereinafter NP). The function of a higher harmonic control (HHC) device is to generate additional airloads on the rotor so as to reduce or cancel the NP vibratory hub load and thus reduce NP vibration in the non-rotating frame of reference.
Various schemes for reducing helicopter vibrations by HHC have been investigated. Some approaches are based on vibration control concepts involving dynamically tuned mechanisms which actuate either the swashplate or tab surfaces on the rotor blade. Other HHC concepts make use of high frequency active control systems which, when coupled with vibration sensors, provide vibration reduction by either manual control or closed loop feedback control.
Reducing rotor-blade induced vibrations in a dual, counter-rotating, coaxial rotor system is further complicated as control inputs to the upper rotor control system and lower rotor control system are typically linked or slaved. As such, HHC systems have heretofore been linked or slaved such that the HHC inputs to the upper rotor system are a fixed multiple of the inputs to the lower rotor system. Such linkage may well be acceptable to reduce vibrations to a certain extent but will not provide the more significant vibration reduction levels demanded by current rotary-wing aircraft operators.
The 1980 American Helicopter Society (AHS) paper entitled “Design of Higher Harmonic Control for the ABC”, J. O'Leary and W. Miao, publicly describes the originally proposed HHC system for an aircraft with a coaxial rotor system. The system does control six “signal” actuators to provide HHC inputs to the main servos i.e., three inputs to the signal actuators inputting into main servo of the upper rotor and three inputs to the signal actuators inputting into the main servos of the lower rotor. In this case, however, the three inputs to the upper signal actuators are fixed multiples of the three inputs to the lower signal actuators such that the upper rotor signal inputs are “slaved” to the lower rotor signal inputs. The pilot flight controls for the upper and lower rotors were also slaved together. Thus, following this slaving philosophy for the HHC inputs was a natural approach. It is apparent in this AHS paper that the analyses projected imperfect vibration control. This is because the slaving process only produces a total of three unique controls whereas there are up to six vibratory hub loads that require suppression for excellent vibration reduction.